High-resolution camera devices such as digital cameras and camera-mounted mobile phones have been widely used. A digital camera generally includes a lens, an image sensor, and a digital signal processor (DSP). The lens condenses light reflected from an object to be photographed, and transmits the light to the image sensor, which detects the condensed light and converts the light into an electric signal. The DSP performs a digital signal processing on the electric signal from the image sensor such that a user may view an image or store the photographed image.
Image sensors typically comprise image pickup tubes and solid image sensors, and a charge-coupled device (CCD) and a complementary metal-oxide semiconductor (CMOS) are the common type of the solid image sensor.
A CCD image sensor is formed of a circuit where several pairs of capacitors are connected to one another, and capacitors transfer electrodes accumulated thereon to an adjacent capacitor. The CCD image sensor or chip includes a plurality of photodiodes, each of which generates electrons according to an amount of light incident thereto. Information generated by the photodiode is reconstructed to form image information.
Since it is possible to manufacture a CMOS image sensor by use of a general-use semiconductor manufacturing apparatus, the CMOS image sensor costs less than a CCD image sensor in terms of manufacturing expense. Accordingly, the CMOS image sensors are widely used for inexpensive digital cameras or television cameras having slow frames. However, in low light, pixels of the CMOS image sensor may become easily unstable and a substantial amount of noise may occur in a photographed image.
With respect to a recently introduced contact image sensor (CIS), deterioration of sensitivity of photographed images may occur as the number of valid pixels increases.